FIG. 2 is a vertical sectional view of a conventional unidirectional (primary sound pressure gradient) condenser microphone unit (see FIG. 3 of Japanese Patent Application Publication No. 2000-83292).
In FIG. 2, a condenser microphone unit 1 comprises a condenser section 2 consisting of a diaphragm 3 and a rear pole plate 4 arranged opposite each other via a spacer having a predetermined thickness. The condenser section 2 is placed close to a front side of a cylindrically formed metal case 9 on which front acoustic terminal holes 9a are located.
The rear pole plate 4 is supported on an insulating pedestal 5 having a rear acoustic terminal hole 5a. The rear acoustic terminal hole 5a is in communication with a through holes 4a formed in the rear pole plate 4, via an acoustic resistance material 5b consisting of a felt material or the like.
An impedance converter (FET) 7 mounted on a printed circuit board 6 is housed in a rear opening in the metal case 9. Open edges 9b of the metal case 9 are caulked so as to be curled inward to allow the printed circuit board 6 to seal the rear opening.
The impedance converter 7 and the rear pole plate 4 are in a configurational relationship in which they are electrically connected together as a result of a relay terminal member 8 interposed between the impedance converter 7 and the rear pole plate 4; the relay terminal member 8 is composed of a combination of a contact spring material 8a and a conductive elastic material 8b. 
In the condenser microphone unit 1, shown in FIG. 2, the impedance converter 7 and the rear pole plate 4 are electrically connected together via the relay terminal member 8, consisting of the contact spring 8a and conductive elastic material 8b. 
However, a conducting path for the rear pole plate 4 is established only by the point contact of the contact spring 8a with the rear pole plate 4. Accordingly, if, for example, sebum adheres to the contact spring 8a, the contact with the rear pole plate 4 becomes unstable. This may result in noise or a decrease in sensitivity.